sjansen/mecha

package main

import "fmt"

func main() {
    f1 := NewFrobnicator(
        WithA(3),
        WithB(4),
        WithAdd(),
    )
    f2 := NewFrobnicator(
        WithA(3),
        WithB(4),
        WithMultiply(),
    )
    fmt.Printf("f1(1)=%-3d f1(2)=%-3d\n", f1.Frob(1), f1.Frob(2))
    fmt.Printf("f2(1)=%-3d f2(2)=%-3d\n", f2.Frob(1), f2.Frob(2))
}

type Frobnicator struct {
    a, b int
    frob func(int, int) int
}

func (f *Frobnicator) Frob(c int) int {
    return f.frob(
        f.frob(c, f.a),
        f.frob(c, f.b),
    )
}

func NewFrobnicator(options ...Option) *Frobnicator {
    f := &Frobnicator{}
    for _, option := range options {
        option(f)
    }
    return f
}

type Option func(f *Frobnicator) *Frobnicator

func WithA(x int) Option {
    return func(f *Frobnicator) *Frobnicator {
        f.a = x
        return f
    }
}

func WithB(x int) Option {
    return func(f *Frobnicator) *Frobnicator {
        f.b = x
        return f
    }
}

func WithAdd() Option {
    return func(f *Frobnicator) *Frobnicator {
        f.frob = func(a, b int) int {
            return a + b
        }
        return f
    }
}

func WithMultiply() Option {
    return func(f *Frobnicator) *Frobnicator {
        f.frob = func(a, b int) int {
            return a * b
        }
        return f
    }
}